It is well known that signals transported over an optical network suffer degradation between associated transmitters and receivers. There are many possible causes for the transmitted optical signals to degrade; among them are polarization mode dispersion (PMD), chromatic dispersion (CD), etc. Some of those effects might change the signal characteristic over time, some effects are temperature dependant. No matter what caused the degradation, the more the received signal is distorted, the more errors will be made at detection up to the point where the transmission becomes ineffective.
There are many ways to compensate for errors at the receiver. For example, forward error correction (FEC) is commonly used in optical transmission networks to correct errors of the received signals. In FEC, the transmitted signals include redundant information used for reconstruction of the transmitted sequence (error correction). Another example is an adaptive receiver that allows for the receiver to adjust and/or modify optical and/or electrical components thereby reducing the amount of errors received at the receiver. A combination of the two examples is also possible.
The adaptive receiver functions well in static optical networks where signals travel substantially the same path. Slow changes in the input signal characteristic at the receiver might be compensated by adjustments done by the adaptive receiver. The input signal characteristics can be compensated as long as the time constants of the adaptive receiver are faster than the signal changes. In particular, if some of the adjustments of the adaptive receiver are based upon the number of errors computed from the overhead information, then the time constants of the adaptive receiver are dependant on the signal quality. In that case, some of the adjustments might start to drift from the optimum for high quality input signals. This drift might be caused either by some offset in the control loop and/or by slowly changing characteristics of the input signal to the optical receiver. This will be termed ‘the receiver is outside the active control region’ henceforth.
However, in an optically switched WDM network where the optical signals are constantly being switched onto different paths, the signal characteristic could change abruptly at the receiver. The latter is not limited to the switched signals but also applies to neighbor channels of the WDM link. The sudden change of signal characteristic will most probably result in drastic increase of errors at the receiver if the adaptive receiver is not at the optimum setting. As a result, the FEC might not be able to compensate for the errors at the receiver. If the settings of the adaptive receiver have drifted from the optimum settings, transmission faults will likely occur.
An object of the present invention is, therefore, to enable the adaptive receiver to find and track the optimum setting thus enhancing the robustness against abrupt signal degradation.